1. Field of the Invention
The invention concerns an apparatus for detecting the presence and/or the size of a cooking vessel on an electrically heated hotplate or hob, which includes a resistance heating element which is disposed under the hotplate or hob and which is connected to an electrical heating voltage supply.
2. Discussion of the Prior Art
Pot detection apparatuses of that kind are widely known. Thus, EP 0 788 293 A1 describes an electrical radiant heating body with a pot detection means, which uses an inductive sensor in the form of a loop of thick wire, which is disposed in the air space between the heating conductor and the hotplate and serves as an inductive element of a resonant circuit whose resonance frequency is displaced when a pot is placed on the hotplate by virtue of a change in the inductance of the wire loop.
DE 196 46 826 A1 discloses a pot detection arrangement using capacitive sensors which are formed by a plurality of electrodes, which are also disposed between the heating conductor and the hotplate, and with which an electrical resonance circuit is driven. When a pot is placed on the hotplate, the capacitance of those sensors changes, and therewith also the resonance frequency of the resonant circuit.
DE 39 34 157 A1 describes a cooking tray or hob in which a plurality of sensors are disposed therein for the purposes of detecting the surface area occupied by cooking vessels of different sizes. The sensors used are ultrasonic sensors or capacitive sensors, for measuring the heat flux or for measuring the radiation issuing from the heating means and the hotplate.
The pot detection apparatus disclosed in EP 0 553 425 B1 uses a sensor in the form of an electrical conductor which forms an open loop and which is disposed between the heating element and the hotplate. When a pot is put onto the hotplate, the conductivity of that sensor changes.
The described state of the art, for pot detection purposes, always requires special sensors which are arranged in the cooking tray or between the heating conductor and the hotplate. Those sensors must be designed in such a way that they can withstand the high temperatures of the cooking zone. In addition, steps must be taken to ensure that those sensors do not interfere with the electrical insulation system of heating means-cooking pot. In addition, a pot detection apparatus with such sensors is not suitable for use in relation to hotplates, against the underside of which the electrical resistance heating bears directly (by way of an insulating layer), without the presence there of an intermediate air gap in which those sensors could be disposed.